I can so relate to your confidence problems
I started riding about 5 years ago, on quite difficult trails because I live in a very hilly area and I have no arena. It went quite well for a while. But then I fell off and hurt myself, so there went my confidence. Time passed and I was always sooo nervous to go back up on my horse, even though she is a very sweet and calm one (maybe a little bit too young to be the best learner).
Last year I started taking lessons with a new girl in the neighbourhood, English lessons on a very calm pony (I used to ride western on a Paint horse). And I have to say that it has helped me a lot.
I would so have loved to start in an arena, and my best advice to you is to ride a lot in the arena. Don't go outside the arena until you are bored and can ride with your eyes closed (so to speak). If you are in a good relationship with your brother-in-law then ask him, I believe that a man would always love to be asked to help a woman with a thing that he is good at
Always wear a helmet (even if you are a western rider). Sing or hum to keep yourself relaxed, take some deep breaths. Don't overdo it, it's better to go often and for a short time, than building up tension and a feeling of "I have to go, I should go, too many days have passed now but I feel nervous". Always end your ride on a positive note, that will build your confidence as well as the horse's.
Best of luck!